The circulatory system consists of the heart, blood vessels, and blood. The heart pumps blood through the network of arteries, veins, and capillaries that make up the blood vessels. Blood carries oxygen, nutrients, hormones, and waste products as it circulates through the double circulatory system. In the lungs, blood picks up oxygen and releases carbon dioxide, and the heart pumps oxygenated blood throughout the body via arteries while bringing deoxygenated blood back to the lungs through veins.